<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
<!--    <meta name="viewport" content="width=device-width, initial-scale=1.0">-->
    <title>Title</title>
    <style>
        * {
            padding: 0;
            margin: 0;
        }

        html {
            font-size: 20px;
        }
        #one{
            width: 30rem;
            height: 30rem;
            background: green;
            font-size: 10px;
        }

        #two{
            width: 20rem;
            height: 20rem;
            background: red;
            font-size: 15px;
        }
        #three{
            width: 10em;
            height: 10em;
            background: yellow;
        }


    </style>
</head>
<body>
    <div id="one">
        <div id="two">
            <div id="three"></div>
        </div>
    </div>
</body>
</html>